1. Field of the Invention
The present invention relates to a process of a semiconductor fabrication system for controlling the distribution of dopants or impurities in a device. More specifically, the present invention relates to a technique for controlling dopant concentrations in a substrate using transient-enhanced diffusion.
2. Description of the Related Art
Conductive regions of N-type and P-type conductivities and N-P junctions at the boundaries of conductive regions are essential for providing electrical functionality of semiconductor devices. The conductive regions and junctions are formed in a semiconductor wafer by diffusion or ion implantation techniques. The N-P and P-N junctions in a semiconductor substrate form structures that provide the electrical functionality of transistors and diodes. A junction is a separation between a region called an N-type region having a high concentration of negative electrons and a region called a P-type region having a high concentration of holes. A junction is typically formed in a semiconductor wafer by thermal diffusion, or ion implantation and annealing.
To produce high density circuits, smaller feature sizes and closer spacing of circuit components must be achieved. Thermal diffusion is generally an insufficient technique for producing compact structures for the fabrication of high-density circuits. Thermal diffusion is insufficient for several reasons including a propensity for diffusion to extend in a lateral direction, poor control of doping concentration, surface contamination interference, excessively thin junctions, and a propensity to generate dislocations. Lateral diffusion occurs during deposition and annealing and continues each time the wafer is heated. Sufficient spacing between adjacent regions must be allowed to prevent contact between regions due to lateral diffusion. Disadvantageously, the ultimate effect of lateral diffusion on a dense circuit is an increased die size.
Many of the problems inherent to thermal diffusion are solved to some extent by implanting dopants into a substrate. Ion implantation solves these problems by directly injecting atoms into a substrate lattice. However, some problems do arise with ion implantation. First, ion implantation causes damage to the substrate lattice. Second, the ions injected into the substrate lattice are not active and require activation. A thermal annealing process is typically employed to repair damage to the substrate structure, activate the implanted ions, and diffuse the implanted ions.
While ion implantation greatly improves control of the dopant concentration within the substrate, additional control is needed to further reduce structure sizes and to improve the precision of dopant concentrations within the substrate, particularly in the vicinity of junctions. These improvements are particularly useful for precisely setting threshold voltage V.sub.T and preventing "punchthrough".